Hydroxyphenyl-piperazinyl-methyl-benzamide derivatives for the treatment of pain

ABSTRACT

The present application describes compounds of general formula I                    
     where R 1  is selected from any one of pyridinyl, thienyl, furanyl, imidazolyl, and triazolyl; 
     and where each R 1  heteroaromatic ring may optionally and independently be further substituted by 1, 2 or 3 substituents selected from straight and branched C 1 -C 6  alkyl, NO 2 , CF 3 , C 1 -C 6  alkoxy, chloro, fluoro, bromo, and iodo. The substitutions on the heteroaromatic ring may take place in any position on the ring system. The invention also includes enantiomers, salts and pharmaceutical compositions containing the compounds. The compounds may be used in treating patients for pain.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application represents U.S. national stage of internationalapplication PCT/SE01/00709, which had an international filing date ofMar. 30, 2001, and which was published in English under PCT Article21(2) on Oct. 11, 2001. The international application claims priority toSwedish application 0001209-6, filed on Apr. 4, 2000.

FIELD OF THE INVENTION

The present invention is directed to novel compounds, to a process fortheir preparation, their use and pharmaceutical compositions comprisingthe novel compounds. The novel compounds are useful in therapy, and inparticular for the treatment of pain.

BACKGROUND AND PRIOR ART

The δ receptor has been identified as having a role in many bodilyfunctions such as circulatory and pain systems. Ligands for the δreceptor may therefore find potential use as analgesics, and/or asantihypertensive agents. Ligands for the δ receptor have also been shownto possess immunomodulatory activities.

The identification of at least three different populations of opioidreceptors (μ, δ and κ) is now well established and all three areapparent in both central and peripheral nervous systems of many speciesincluding man. Analgesia has been observed in various animal models whenone or more of these receptors has been activated.

With few exceptions, currently available selective opioid δ ligands arepeptidic in nature and are unsuitable for administration by systemicroutes. One example of a non-peptidic δ-agonist is SNC80 (Bilsky E. J.et al., Journal of Pharmacology and Experimental Therapeutics, 273(1),pp. 359-366 (1995)). There is however still a need for selectiveδ-agonists having not only improved selectivity, but also an improvedside-effect profile.

Thus, the problem underlying the present invention was to find newanalgesics having improved analgesic effects, but also with an improvedside-effect profile over current μ agonists, as well as having improvedsystemic efficacy.

Analgesics that have been identified and are existing in the prior arthave many disadvantages in that they suffer from poor pharmacokineticsand are not analgesic when administered by systemic routes. Also, it hasbeen documented that preferred δ agonist compounds, described within theprior art, show significant convulsive effects when administeredsystemically.

We have now found that certain compounds not specifically disclosed by,but included within the scope of WO 97/23466, exhibit surprisinglyimproved δ-agonist properties and in vivo potency.

OUTLINE OF THE INVENTION

The novel compounds according to the present invention are defined bythe formula I

wherein

R¹ is selected from any one of

where each R¹ heteroaromatic ring may optionally and independently befurther substituted by 1, 2 or 3 substituents selected from straight andbranched C₁-C₆ alkyl, NO₂, CF₃, C₁-C₆ alkoxy, chloro, fluoro, bromo, andiodo. The substitutions on the heteroaromatic ring may take place in anyposition on said ring systems;

A preferred embodiment of the present invention is a compound accordingto FIG. 1 wherein R¹ is as defined above and each R¹ phenyl ring and R¹heteroaromatic ring may independently be further substituted by a methylgroup.

A more preferred embodiment of the present invention is a compoundaccording to FIG. 1 wherein R₁ is pyridinyl, thienyl or furanyl.

Within the scope of the invention are also separate enantiomers andsalts of the compounds of the formula I, including salts of enantiomers.

Separation of racemic mixtures into separate enantiomers is well knownin the art and may be accomplished for example by separation on asuitable chiral chromatography column. Preparation of salts is wellknown in the art, and may be accomplished for example by mixing acompound of formula I in a suitable solvent with the desired protic acidand isolation by means standard in the art. Salts of compounds offormula I include pharmaceutically acceptable salts and alsopharmaceutically unacceptable salts.

When the heteroaromatic ring(s) are substituted, the preferredsubstituents are selected from anyone of CF₃, methyl, iodo, bromo,fluoro and chloro.

Key reaction step Scheme 1, vide infra, is performed by reacting anintermediate compound of the general formula II

wherein R is an N-protecting group such as Boc or CBz, and P is aO-protecting group such as TBS or Me, which is first N-deprotected andthen alkylated using either,

i) a compound of the general formula R¹—CH₂—X, wherein R¹ is as definedabove and X is halogen, preferably bromide, and a suitable base, or

ii) a compound of the general formula R¹—CHO, wherein R¹ is as definedabove, and a suitable reducing agent to give the compounds of generalformula I, after O-deprotection.

Suitable bases to be used in the standard alkylation step i) aboveincludes, but is not limited to, triethylamine and potassium carbonate.

Suitable reducing agents to be used in the standard reduction step ii)includes, but is not limited to, sodium cyanoborohydride and sodiumtriacetoxyborohydride.

The novel compounds of the present invention are useful in therapy,especially for the treatment of various pain conditions such as chronicpain, neuropathic pain, acute pain, cancer pain, pain caused byrheumatoid arthritis, migraine, visceral pain etc. This list shouldhowever not be interpreted as exhaustive.

Compounds of the invention are useful as immunomodulators, especiallyfor autoimmune diseases, such as arthritis, for skin grafts, organtransplants and similar surgical needs, for collagen diseases, variousallergies, for use as anti-tumour agents and anti viral agents.

Compounds of the invention are useful in disease states wheredegeneration or dysfunction of opioid receptors is present or implicatedin that paradigm. This may involve the use of isotopically labelledversions of the compounds of the invention in diagnostic techniques andimaging applications such as positron emission tomography (PET).

Compounds of the invention are useful for the treatment of diarrhoea,depression, anxiety, urinary incontinence, various mental illnesses,cough, lung oedema, various gastro-intestinal disorders, spinal injuryand drug addiction, including the treatment of alcohol, nicotine, opioidand other drug abuse and for disorders of the sympathetic nervous systemfor example hypertension.

Compounds of the invention are useful as an analgesic agent for useduring general anaesthesia and monitored anaesthesia care. Combinationsof agents with different properties are often used to achieve a balanceof effects needed to maintain the anaesthetic state (eg. amnesia,analgesia, muscle relaxation and sedation). Included in this combinationare inhaled anaesthetics, hypnotics, anxiolytics, neuromuscular blockersand opioids.

Also within the scope of the invention is the use of any of thecompounds according to the formula I above, for the manufacture of amedicament for the treatment of any of the conditions discussed above.

A further aspect of the invention is a method for the treatment of asubject suffering from any of the conditions discussed above, whereby aneffective amount of a compound according to the formula I above, isadministered to a patient in need of such treatment.

A further aspect of the present invention is intermediates of thegeneral formula II,

wherein R is an N-protecting group such as Boc or CBz, and P is aO-protecting group such as TBS or Me.

Methods of Preparation

The compounds according to the present invention may be prepared byfollowing any one of the procedures described in Schemes I, II, III andIV below. Similar procedures are described in J. March, Advanced OrganicChemistry, 4^(th) Edition, John Wiley and sons (1992); Katritsky, A. R.,Lan, X Chem. Soc. Rev., pp. 363-373 (1994), which are herebyincorporated by reference.

EXAMPLES

The invention will now be described in more detail by the followingExamples, which are not to be construed as limiting the invention.

Example 1

Preparation ofN,N-diethyl-4-[[4-(2-furylmethyl)-1-piperazinyl](3-hydroxyphenyl)methyl]benzamide(Compound 7)

(i) Preparation of 3-{[tert-butyl(dimethyl)silyl]oxy}benzaldehyde(Compound 2)

3-Hydroxybenzaldehyde (10 g, 82 μmmol) was dissolved in DMF (50 mL) withimidazole (12 g, 180 mmol) and t-butyldimethylsilyl chloride (13 g, 90mmol) and stirred at 25° C. for 12 h. Aqueous workup and chromatographyon silica gave compound 2 (18 g, 93%).

MS (EI) m/e 236, 179, 151.

(ii) Preparation of4-[(3-{[tert-butyl(dimethyl)silyl]oxy}phenyl)(hydroxy)methyl]-N,N-diethylbenzamide(Compound 4)

N,N-Diethyl-4-iodobenzamide (3.0 g, 10 mmol) was dissolved in THF (100mL) and cooled to −78° C. under nitrogen atmosphere. n-BuLi (7.7 mL, 1.3M solution in hexane, 10 mmol) was added dropwise during 10 min at −65to −78° C. Compound 2 prepared in the previous step (1.9 g, 8.0 mmol)was added dropwise dissolved in THF (2 mL). NH₄Cl (aq.) was added after30 min. After concentration in vacuo, extraction with EtOAc/water,drying (MgSO₄) and evaporation of the organic phase, the residue waspurified by chromatography on silica to give compound 4 (2.0 g, 60%).

¹H NMR (CDCl₃) δ 0 (s, 6H), 0.80 (s, 9H), 0.9-1.2 (m, 6H), 2.6 (s, 1H),3.0-3.5 (m, 4H), 5.59 (s, 1H), 6.55-7.25 (m, 8H).

(iii) Preparation of4-[(3-{[tert-butyl(dimethyl)silyl]oxy}phenyl)(1-piperazinyl)methyl]-N,N-diethylbenzamide(Compound 5)

Compound 4 prepared in the previous step (2.0 g, 4.8 mmol) was dissolvedin dry CH₂Cl₂ (50 mL) and treated with SOCl₂ (0.38 mL, 5.2 mmol) in at 0to 25° C. for 30 min, the solvent was evaporated in vacuo. The residuewas dissolved in MeCN (50 mL) and reacted with piperazine (1.6 g, 19mmol) at 80° C. for 12 h. Concentration in vacuo and chromatography onsilica gave compound 5 (1.2 g, 52%).

¹H NMR (amine, CDCl₃) δ=1.0, 1.1 (2m, 6H), 2.2-2.4 (m, 4H), 2.80 (m,4H), 3.15, 3.45 (2m, 4H), 4.10 (s, 11H), 6.58-7.38 (m, 8H).

(iv)4-{(3-{[tert-butyl(dimethyl)silyl]oxy}phenyl)[4-(2-furylmethyl)-1-piperazinyl]methyl}-N,N-diethylbenzamide(Compound 6)

2-furylmethanol (compound 3 prepared as shown in Scheme 1 above) (0.29mL, 3.3 mmol) was dissolved in CH₂Cl₂ (5 mL) with triethylamine (1.0 mL,7.4 mmol). Methanesulfonylchloride (0.26 mL, 3.4 mmol) was added at 0°C. and the solution stirred 15 min at 0° C. before compound 5 preparedin the previous step (0.81 g, 1.7 mmol) was added. The reaction wasstirred 48 h at 25° C. before concentration in vacuo and chromatographyon silica gave compound 6 (0.40 g, 42%).

(v) Preparation ofN,N-diethyl-4-[[4-(2-furylmethyl)-1-piperazinyl](3-hydroxyphenyl)methyl]benzamide(Compound 7)

Compound 6 prepared in the previous step was treated with DMF/2N HCl,1:2 for 1 h at 25° C. Concentration and reverse phase chromatographygave the title compound 7 (0.26 g, 54%) as the ditrifluoroacetate salt.

MS (ES) 448.24 (MH+monoisot.).

IR (2×TFA, NaCl): 3232, 1674, 1599, 1457, 1288,1199, 1134 (cm-1).

¹H NMR (2×TFA, CDCl₃) δ=1.1, 1.2 (2m, 6H), 2.5-3.6 (m, 13H), 4.21 (s,2H), 4.28 (s, 1H), 6.44-7.50 (m, 11H).

Example 2

Preparation ofN,N-diethyl-4-{(3-hydroxyphenyl)[4-(2-thienylmethyl)-1-piperazinyl]methyl}benzamideDitrifluoroacetate (Compound 11)

Prepared as in Example Iiv and Iv from compound 5 as prepared in ExampleIiii above, to give the title compound 11, after purification by reversephase chromatography and extraction (CH₂Cl₂/K₂CO₃(aq)), as base (28 mg,31%).

MS (ES) 464.10 (MH+monoisot.).

IR (2×TFA, NaCl): 3393, 3180, 1672,1607, 1457, 1289,1199, 1133 (cm-1).

¹H NMR (2×HCl, CDC13) δ=1.1, 1.3 (m, 6H), 2.4-3.6 (m, 13H), 4.27 (s,1H), 4.35 (s, 2H), 6.64-7.45 (m, 11H).

Example 3

Preparation ofN,N-diethyl-4-{(3-hydroxyphenyl)[4-(3-thienylmethyl)-1-piperazinyl]methyl}benzamideDihydrochlodride (Compound 9)

Prepared as in Example Iiv and Iv from compound 5 as prepared in ExampleIiii above, to give the title compound 8, after purification by reversephase chromatography and extraction (CH₂Cl₂/K₂CO₃(aq)), as base (29 mg,52%). Treatment with HCl (aq.) gave dihydrochloride. MS (ES) 464.08(MH+monoisot.).

IR (2×HCl, NaCl): 3393, 3180, 1607, 1457, 1289 (cm-1).

¹H NMR (2×HCl, CDCl₃) δ=1.1, 1.2 (m, 6H), 1.6-2.2 (m, 8H), 3.1-4.4 (m,5H), 4.30 (s, 2H), 5.0 (s, 1H), 6.8-7.9 (m, 11H).

Pharmaceutical Compositions

The novel compounds according to the present invention may beadministered orally, intramuscularly, subcutaneously, topically,intranasally, intraperitoneally, intrathoracially, intravenously,epidurally, intrathecally, intracerebroventricularly and by injectioninto the joints.

A preferred route of administration is orally, intravenously orintramuscularly.

The dosage will depend on the route of administration, the severity ofthe disease, age and weight of the patient and other factors normallyconsidered by the attending physician, when determining the individualregimen and dosage level as the most appropriate for a particularpatient.

For preparing pharmaceutical compositions from the compounds of thisinvention, inert, pharmaceutically acceptable carriers can be eithersolid or liquid. Solid form preparations include powders, tablets,dispersible granules, capsules, cachets, and suppositories.

A solid carrier can be one or more substances which may also act asdiluents, flavoring agents, solubilizers, lubricants, suspending agents,binders, or tablet disintegrating agents; it can also be anencapsulating material.

In powders, the carrier is a finely divided solid which is in a mixturewith the finely divided active component. In tablets, the activecomponent is mixed with the carrier having the necessary bindingproperties in suitable proportions and compacted in the shape and sizedesired.

For preparing suppository compositions, a low-melting wax such as amixture of fatty acid glycerides and cocoa butter is first melted andthe active ingredient is dispersed therein by, for example, stirring.The molten homogeneous mixture is then poured into convenient sizedmolds and allowed to cool and solidify.

Suitable carriers are magnesium carbonate, magnesium stearate, talc,lactose, sugar, pectin, dextrin, starch, tragacanth, methyl cellulose,sodium carboxymethyl cellulose, a low-melting wax, cocoa butter, and thelike.

Salts include, but are not limited to pharmaceutically acceptable salts.Examples of pharmaceutically acceptable salts within the scope of thepresent invention include: acetate, benzenesulfonate, benzoate,bicarbonate, bitartrate, bromide, calcium acetate, camsylate, carbonate,chloride, citrate, dihydrochloride, edetate, edisylate, estolate,esylate, fumarate, glucaptate, gluconate, glutamate,glycollylarsanilate, hexylresorcinate, hydrabamine, hydrobromide,hydrochloride, hydroxynaphthoate, isethionate, lactate, lactobionate,malate, maleate, mandelate, mesylate, methylbromide, methylnitrate,methylsulfate, mucate, napsylate, nitrate, pamoate (embonate),pantothenate, phosphate/diphosphate, polygalacturonate, salicylate,stearate, subacetate, succinate, sulfate, tannate, tartrate, teoclate,triethiodide, benzathine, chloroprocaine, choline, diethanolamine,ethylenediamine, meglumine, procaine, aluminium, calcium, lithium,magnesium, potassium, sodium, and zinc. Examples of pharmaceuticallyunacceptable salts within the scope of the present invention include:hydroiodide, perchlorate, and tetrafluoroborate. Preferredpharmaceutically acceptable salts are the hydrochlorides, sulfates andbitartrates. The hydrochloride and sulfate salts are particularlypreferred.

The term composition is intended to include the formulation of theactive component with encapsulating material as a carrier providing acapsule in which the active component (with or without other carriers)is surrounded by a carrier which is thus in association with it.Similarly, cachets are included.

Tablets, powders, cachets, and capsules can be used as solid dosageforms suitable for oral administration.

Liquid from compositions include solutions, suspensions, and emulsions.Sterile water or water-propylene glycol solutions of the activecompounds may be mentioned as an example of liquid preparations suitablefor parenteral administration. Liquid compositions can also beformulated in solution in aqueous polyethylene glycol solution.

Aqueous solutions for oral administration can be prepared by dissolvingthe active component in water and adding suitable colorants, flavoringagents, stabilizers, and thickening agents as desired. Aqueoussuspensions for oral use can be made by dispersing the finely dividedactive component in water together with a viscous material such asnatural synthetic gums, resins, methyl cellulose, sodium carboxymethylcellulose, and other suspending agents known to the pharmaceuticalformulation art.

Preferably the pharmaceutical compositions is in unit dosage form. Insuch form, the composition is divided into unit doses containingappropriate quantities of the active component. The unit dosage form canbe a packaged preparation, the package containing discrete quantities ofthe preparations, for example, packeted tablets, capsules, and powdersin vials or ampoules. The unit dosage form can also be a capsule,cachet, or tablet itself, or it can be the appropriate number of any ofthese packaged forms.

Biological Evaluation

In Vitro Model

Cell Culture

A. Human 293S cells expressing cloned human μ, δ, and κ receptors andneomycin resistance were grown in suspension at 37° C. and 5% CO₂ inshaker flasks containing calcium-free DMEM10% FBS, 5% BCS, 0.1% PluronicF-68, and 600 μg/ml geneticin.

B. Mouse and rat brains were weighed and rinsed in ice-cold PBS(containing 2.5 mM EDTA, pH 7.4). The brains were homogenized with apolytron for 15 sec (mouse) or 30 sec (rat) in ice-cold lysis buffer (50mM Tris, pH 7.0, 2.5 mM EDTA, with phenylmethylsulfonyl fluoride addedjust prior use to 0.5MmM from a 0.5M stock in DMSO:ethanol).

Membrane Preparation

Cells were pelleted and resuspended in lysis buffer (50 mM Tris, pH 7.0,2.5 mM EDTA, with PMSF added just prior to use to 0.1 mM from a 0.1 Mstock in ethanol), incubated on ice for 15 min, then homogenized with apolytron for 30 sec. The suspension was spun at 1000 g (max) for 10 minat 4° C. The supernatant was saved on ice and the pellets resuspendedand spun as before. The supernatants from both spins were combined andspun at 46,000 g(max) for 30 min. The pellets were resuspended in coldTris buffer (50 mM Tris/Ci, pH 7.0) and spun again. The final pelletswere resuspended in membrane buffer (50 mM Tris, 0.32 M sucrose, pH7.0). Aliquots (1 ml) in polypropylene tubes were frozen in dryice/ethanol and stored at −70° C. until use. The protein concentrationswere determined by a modified Lowry assay with sodium dodecyl sulfate.

Binding Assays

Membranes were thawed at 37° C., cooled on ice, passed 3 times through a25-gauge needle, and diluted into binding buffer (50 mM Tris, 3 mMMgCl₂, 1 mg/ml BSA (Sigma A-7888), pH 7.4, which was stored at 4° C.after filtration through a 0.22 m filter, and to which had been freshlyadded 5 μg/ml aprotinin, 10 μM bestatin, 10 μM diprotin A, no DTT).Aliquots of 100 μl were added to iced 12×75 mm polypropylene tubescontaining 100 μl of the appropriate radioligand and 100 μl of testcompound at various concentrations. Total (TB) and nonspecific (NS)binding were determined in the absence and presence of 10 μM naloxonerespectively. The tubes were vortexed and incubated at 25° C. for 60-75min, after which time the contents are rapidly vacuum-filtered andwashed with about 12 ml/tube iced wash buffer (50 mM Tris, pH 7.0, 3 mMMgCl₂) through GF/B filters (Whatman) presoaked for at least 2 h in 0.1%polyethyleneimine. The radioactivity (dpm) retained on the filters wasmeasured with a beta counter after soaking the filters for at least 12 hin minivials containing 6-7 ml scintillation fluid. If the assay is setup in 96-place deep well plates, the filtration is over 96-placePEI-soaked unifilters, which were washed with 3×1 ml wash buffer, anddried in an oven at 55° C. for 2 h. The filter plates were counted in aTopCount (Packard) after adding 50 μl MS-20 scintillation fluid/well.

Functional Assays

The agonist activity of the compounds is measured by determining thedegree to which the compounds receptor complex activates the binding ofGTP to G-proteins to which the receptors are coupled. In the GTP bindingassay, GTP[γ]³⁵S is combined with test compounds and membranes fromHEK-293S cells expressing the cloned human opioid receptors or fromhomogenised rat and mouse brain. Agonists stimulate GTP[γ]³⁵ S bindingin these membranes. The EC₅₀ and E_(max) values of compounds aredetermined from dose-response curves. Right shifts of the dose responsecurve by the delta antagonist naltrindole are performed to verify thatagonist activity is mediated through delta receptors.

Data Analysis

The specific binding (SB) was calculated as TB-NS, and the SB in thepresence of various test compounds was expressed as percentage ofcontrol SB. Values of IC₅₀ and Hill coefficient (nH) for ligands indisplacing specifically bound radioligand were calculated from logitplots or curve fitting programs such as Ligand, GraphPad Prism,SigmaPlot, or ReceptorFit. Values of K_(i) were calculated from theCheng-Prussoff equation. Mean±S.E.M. values of IC₅₀, K_(i) and n_(H)were reported for ligands tested in at least three displacement curves.Biological data are tabulated on the following page in Table 1.

TABLE 1 Biological data. HDELTA RAT BRAIN MOUSE BRAIN Ex. # MOLECULARSTRUCTURE HDELTA EC50 % Emax EC50 % EMax EC50 % Emax 1

0.381 0.21 94.84 1.25 130.09 2.6 112.93 2

0.357 0.24 104.62 0.79 120.37 1.16 117.56 3

0.338 0.15 114.45 0.93 128.08 1 122.03

Receptor Saturation Experiments

Radioligand K_(δ) values were determined by performing the bindingassays on cell membranes with the appropriate radioligands atconcentrations ranging from 0.2 to 5 times the estimated K_(δ) (up to 10times if amounts of radioligand required are feasible). The specificradioligand binding was expressed as pmole/mg membrane protein. Valuesof K_(δ) and B_(max) from individual experiments were obtained fromnonlinear fits of specifically bound (B) vs. nM free (F) radioligandfrom individual according to a one-site model.

Determination of Mechano-Allodynia using Von Frey Testing

Testing was performed between 08:00 and 16:00 h using the methoddescribed by Chaplan et al. (1994). Rats were placed in Plexiglas cageson top of a wire mesh bottom which allowed access to the paw, and wereleft to habituate for 10-15 min. The area tested was the mid-plantarleft hind paw, avoiding the less sensitive foot pads. The paw wastouched with a series of 8 Von Frey hairs with logarithmicallyincremental stiffness (0.41, 0.69, 1.20, 2.04, 3.63, 5.50, 8.51, and15.14 grams; Stoelting, Ill, USA). The von Frey hair was applied fromunderneath the mesh floor perpendicular to the plantar surface withsufficient force to cause a slight buckling against the paw, and heldfor approximately 6-8 seconds. A positive response was noted if the pawwas sharply withdrawn. Flinching immediately upon removal of the hairwas also considered a positive response. Ambulation was considered anambiguous response, and in such cases the stimulus was repeated.

Testing Protocol

The animals were tested on postoperative day 1 for the FCA-treatedgroup. The 50% withdrawal threshold was determined using the up-downmethod of Dixon (1980). Testing was started with the 2.04 g hair, in themiddle of the series. Stimuli were always presented in a consecutiveway, whether ascending or descending. In the absence of a paw withdrawalresponse to the initially selected hair, a stronger stimulus waspresented; in the event of paw withdrawal, the next weaker stimulus waschosen. Optimal threshold calculation by this method requires 6responses in the immediate vicinity of the 50% threshold, and countingof these 6 responses began when the first change in response occurred,e.g. the threshold was first crossed. In cases where thresholds felloutside the range of stimuli, values of 15.14 (normal sensitivity) or0.41 (maximally allodynic) were respectively assigned. The resultingpattern of positive and negative responses was tabulated using theconvention, X=no withdrawal; O=withdrawal, and the 50% withdrawalthreshold was interpolated using the formula:

50% g threshold=10^((Xf+kδ))/10,000

where Xf=value of the last von Frey hair used (log units); k=tabularvalue (from Chaplan et al. (1994)) for the pattern of positive/negativeresponses; and δ=mean difference between stimuli (log units). Hereδ=0.224.

Von Frey thresholds were converted to percent of maximum possible effect(% MPE), according to Chaplan et al. 1994. The following equation wasused to compute % MPE:${\% \quad {MPE}} = {\frac{{{Drug}\quad {treated}\quad {threshold}\quad (g)} - {{allodynia}\quad {threshold}\quad (g)}}{{{Control}\quad {threshold}\quad (g)} - {{allodynia}\quad {threshold}\quad (g)}} \times 100}$

Administration of Test Substance

Rats were injected (subcutaneously, intraperitoneally, intravenously ororally) with a test substance prior to von Frey testing, the timebetween administration of test compound and the von Frey test varieddepending upon the nature of the test compound.

Writhing Test

Acetic acid will bring abdominal contractions when administeredintraperitoneally in mice. These will then extend their body in atypical pattern. When analgesic drugs are administered, this describedmovement is less frequently observed and the drug selected as apotential good candidate.

A complete and typical Writhing reflex is considered only when thefollowing elements are present: the animal is not in movement; the lowerback is slightly depressed; the plantar aspect of both paws isobservable. In this assay, compounds of the present inventiondemonstrate significant inhibition of writhing responses after oraldosing of 1-100 μmol/kg.

(i) Solutions Preparation

Acetic acid (AcOH): 120 μL of Acetic Acid is added to 19.88 ml ofdistilled water in order to obtain a final volume of 20 ml with a finalconcentration of 0.6% AcOH. The solution is then mixed (vortex) andready for injection.

Compound (drug): Each compound is prepared and dissolved in the mostsuitable vehicle according to standard procedures.

(ii) Solutions Administration

The compound (drug) is administered orally, intraperitoneally (i.p.),subcutaneously (s.c.) or intravenously (i.v.)) at 10 ml/kg (consideringthe average mice body weight) 20, 30 or 40 minutes (according to theclass of compound and its characteristics) prior to testing. When thecompound is delivered centrally: Intraventricularly (i.c.v.) orintrathecally (i.t.) a volume of 5 μL is administered.

The AcOH is administered intraperitoneally (i.p.) in two sites at 10ml/kg (considering the average mice body weight) immediately prior totesting.

(iii) Testing

The animal (mouse) is observed for a period of 20 minutes and the numberof occasions (Writhing reflex) noted and compiled at the end of theexperiment. Mice are kept in individual “shoe box” cages with contactbedding. A total of 4 mice are usually observed at the same time: onecontrol and three doses of drug.

What is claimed is:
 1. A compound of the formula I

wherein R¹ selected from any one of

where each R¹ heteroaromatic ring may independently be furthersubstituted by 1, 2 or 3 substituents selected from straight andbranched C₁-C₆ alkyl, NO₂, CF₃, C₁-C₆ alkoxy, chloro, fluoro, bromo, andiodo, as well as enantiomer and salts and thereof.
 2. A compoundaccording to claim 1, wherein each R¹ heteroaromatic ring mayindependently be further substituted by 1, 2 or 3 substituents selectedfrom methyl, CF₃, chloro, fluoro, bromo, and iodo.
 3. A compoundaccording to claim 1, wherein said 1, 2 or 3 substituents on each R¹heteroaromatic ring are methyl groups.
 4. A compound according to claim1, wherein R¹ is pyridinyl, thienyl or furanyl.
 5. A compound selectedfrom the group consisting of:N,N-diethyl-4-[[4-(2-furylmethyl)-1-piperazinyl](3-hydroxyphenyl)methyl]benzamide;N,N-diethyl-4-{(3-hydroxyphenyl)[4-(2-thienylmethyl)-1-piperazinyl]methyl}benzamide;andN,N-diethyl-4-{(3-hydroxyphenyl)[4-(3-thienylmethyl)-1-piperazinyl]methyl}benzamide.6. A compound according to any one of claims 1-5, wherein said compoundis in the form of its hydrochloride, dihydrochloride, sulfate, tartrate,ditrifluoroacetate or citrate salts.
 7. A process for preparing acompound of formula I, comprising: a) reacting a compound of formula II

wherein R is an N-protecting group, and P is an O-protecting groupselected from TBS or Me, to remove said N-protecting group: b)alkylating the N-deprotected compound produced in step a) using either:i) a compound of formula R¹—CH₂—X, wherein R¹ is as defined in claim 1and X is a halogen, and a suitable base, or ii) a compound of formulaR¹—CHO, wherein R¹ is as defined in claim 1, and a suitable reducingagent; and c) removing said O-protecting group to give said compound offormula I.
 8. The compound of claim 1, wherein R¹ is a pyridinyloptionally substituted by 1, 2 or 3 substituents selected from: astraight or branched C₁-C₆ alkyl; NO₂; CF₃; C₁-C₆ alkoxy; chloro;fluoro; bromo; and iodo; as well as an enantiomers and salts thereof. 9.The compound of claim 1, wherein R¹ is a thienyl optionally substitutedby 1, 2 or 3 substituents selected from: a straight or branched C₁-C₆alkyl; NO₂; CF₃; C₁-C₆ alkoxy; chloro; fluoro; bromo; and iodo; as wellas an enantiomers and salts thereof.
 10. The compound of claim 1,wherein R¹ is a furanyl optionally substituted by 1, 2 or 3 substituentsselected from a straight or branched C₁-C₆ alkyl; NO₂; CF₃; C₁-C₆alkoxy; chloro; fluoro; bromo; and iodo; as well as an enantiomers andsalts thereof.
 11. The compound of claim 1, wherein R¹ is a imidazolyloptionally substituted by 1, 2 or 3 substituents selected from: astraight or branched C₁-C₆ alkyl; NO₂; CF₃; C₁-C₆ alkoxy; chloro;fluoro; bromo; and iodo; as well as an enantiomers and salts thereof.12. The compound of claim 1, wherein R¹ is a triazolyl optionallysubstituted by 1, 2 or 3 substituents selected from: a straight orbranched C₁-C₆ alkyl; NO₂; CF₃; C₁-C₆ alkoxy; chloro; fluoro; bromo; andiodo; as well as an enantiomers and salts thereof.
 13. The compound ofany one of claims 8-11, wherein said R¹ is either unsubstituted orsubstituted by 1, 2 or 3 methyl groups.
 14. A pharmaceutical compositioncomprising a compound according to any one of claims 1-5 as an activeingredient, together with a pharmacologically and pharmaceuticallyacceptable carrier.
 15. A method for treating a subject for comprisingadministering to said subject an effective amount of a compoundaccording to any one of claims 1-5.
 16. A pharmaceutical compositioncomprising the compound of any one of claims 8-11 as an activeingredient together with a pharmacologically and pharmaceuticallyacceptable carrier.
 17. A method for treating a subject for paincomprising administering to said subject an effective amount of thecompound of any one of claims 8-11.
 18. A compound of formula II

wherein R is an N-protecting group selected from Boc or CBz, and P is anO-protecting group selected from TBS or Me.